The present invention relates to an apparatus and method of manufacture for wafer probe station systems and the use of guarding and shielding systems for limiting electrical leakage currents and noise. More particularly, the invention relates to approaches for providing a chuck apparatus system and a probe assembly which facilitate guarding and shielding techniques for improving the accuracy of low current and low voltage measurements of a device-under-test (DUT), typically a wafer containing one or more integrated circuits.
Modern wafer probe stations have been developed for making accurate low voltage and low current measurements of semiconductor integrated circuit wafers and other electronic component applications. Wafer probe stations having a guarding system have been developed for reducing current leakage, with Kelvin connection systems and the like to eliminate voltage losses associated with conductive line resistances, and electromagnetic interference (EMI) shielding elements for minimizing the effects of parasitic capacitance and noise in the test environment. The technique of guarding to minimize current leakage during low current measurements, the use of Kelvin connections for low voltage measurements, and the provision of EMI shielding portions are well known and discussed extensively in the technical literature. In guarding applications, in particular, an isolated conductor surrounding or otherwise positioned closely adjacent to low current circuitry, and maintained at the same or nearly the same potential provided as the low current circuit conductors, reduces leakage currents such that the low current measurements may be made accurately. In shielding applications, conductive material connected to ground potential reduce the effects of EMI from external and probe station electronics and other noise on test measurements.
The need to observe device behavior with very low level current and voltage measurements is being driven by the ongoing reduction in the integrated circuit semiconductor device geometry in order to increase circuit density, facilitate higher speeds, and reduce power consumption. Decreasing the scale of the circuit can provide the aforementioned improvements, however, tradeoffs in performance may also occur. A number of factors can adversely affect low level voltage and current measurements, including, impedances in which an impedance or current path unintentionally shares a noise source or other instrumentation, the transfer of a noise voltage through usually coupled incidental inductances, magnetically coupled noise, incidental capacitive coupling, charge transfer due to the proximity of charge bodies to the test circuitry, and the like. These mechanisms often perturb measurements taken in integrated circuit devices requiring very low level measurements. The measurement of current values in the high attoampere and the low femtoampere regime is particularly difficult in the presence of interfering sources that may be capable of generating current flow of electrons which, though minuscule, may be substantial relative to the very low voltage and low currents being measured.
In one known approach to providing a guarded and shielded chuck assembly, the assembly includes multiple conductive chuck elements spaced vertically and electrically insulated from each other. The upper chuck element supports the test wafer, and a conductive ring mechanically attached to one of the lower chuck elements surrounds the outer periphery of the chuck assembly to serve as a guard element. In such known assembly, an annular air gap between the chuck assembly elements and the surrounding guard ring serves as a dielectric to isolate the guard ring from the conductive wafer support element. A dielectric material may also be present in the annular gap. The size of the annular space provided in such a design directly affects its dielectric properties and capacitance, and in turn the degree of isolation from the support surface on which testing occurs. However, maintaining the desired registration between the chuck elements and the guard ring in such a design may be difficult. Even slight offsets in the associated mechanical connections between the various elements or in the shape of the guard ring can affect the registration and detrimentally alter the performance of the chuck.
Another known approach involves use of a chuck assembly in which the wafer support layer is a first conductive material sputtered on the upper surface of an insulator element, which in turn rests atop a second conductive chuck element. An electrically isolated dish has a bottom portion which extends laterally below the second conductive element, and an annular side wall which extends around the outer periphery of the chuck assembly and terminates vertically opposite the insulator element. The dish may be connected as a shield and the second conductive element as a guard. Such an approach may be suitable in certain applications, but does not provide significant guarding around the side periphery of the conductive support surface and the location of testing. In addition, with the annular side wall of the shield opposing the metal sputtered insulator element, parasitic and parallel capacitance may occur between the shield and the conductive test surface and distort test measurements.
Probe stations commonly include at least one manipulator that sits on the probe station platen and supports a probe holder, which is typically a metal shaft, either straight or bent, that holds the probe tip on one end and is held by the manipulator on the other. The probe tip is the part of the unit that actually touches the device under test. Both probe holders having built in tips and others using changeable or disposable tips have been developed. Several coaxial and triaxial probe assemblies are available for making low voltage or low current measurements. In a triaxial set up, the probe tip is connected to the center conductor of the triaxial cable, a middle conductor extending along the probe holder is driven as a guard and an outer shield conductor is referenced to ground. Such probe assemblies have been used for applications such as measuring device voltage and current, characterization of bi-polar and FET devices, and characterization of high speed devices.
One known triaxial probe assembly uses a conductive needle tip that is removably attached to the forward end of a horizontally extending probe holder for positioning the needle to engage the DUT. The needle projects at an angle to the longitudinal axis as it extends through an angled passageway in the holding portion. The tip is held in position via a set screw inserted into an internally threaded bore that opens to the forward end of the holding portion for pushing the needle against the passageway wall and clamping it against sliding movement.
One problem with the above-described arrangement is that there are competing considerations between using a set screw that is large enough to avoid stripping the screw threads while keeping the size of the holding portion including the set screw to a minimum for fitting the holding portion under a microscope so as not to obscure the line-of-sight to the area between the tip of the needle and DUT and for providing sufficient room to manipulate the probe tip in the area around the DUT, particularly where other probes are simultaneously being used on the same DUT. In practice, the holding portion is larger than desired and the set screw is still fairly small so that manipulation thereof has been found to be difficult.
Another problem is that clamping the very thin needle can create undue stresses on the needle shaft such as where the screw may cause small indentations or surface irregularities to form. It is these points where stress concentrations can occur leading to needle failure and requiring a time-consuming and tedious needle change-out operation, not to mention the loss of the cost of the broken tip.
Because of the precision placements of the tip that are required, it is essential that the needle be held firmly against shifting during manipulation thereof. With the small set screw and the corresponding small threaded bore described above, the tolerances have to be very tight to ensure that any play between the interengaging threads that may cause there to be less than a highly rigid fixturing of the needle be avoided. Accordingly, the use of a set screw to hold the needle introduces several problems both in operation of the probe as well as in its manufacture, particularly with respect to forming the screw and threaded bore to the desired tolerances.
Electrically, the above-described probe assembly also faces difficulties relating to the ability to optimize the shielding and guarding of the center conductor of the triaxial cable. Immediately rearwardly adjacent the conductive holding portion is a ceramic coupling collar through which the center conductor extends for electrically connecting to the needle tip. The guarding and shielding conductors do not extend to the forward holding portion as the outer shielding conductor abuts against the back end of the ceramic collar and the guarding conductor extends just slightly into the collar. As is apparent, because the shielding and guarding conductors stop well short of the terminal probe tip end of the center conductor, they do not provide the protection against common impedances, incidental capacitive coupling, charge transfer, incidental mutual inductances, magnetically-coupled noise, intrinsic noise sources and straight capacitance charging at the probe tip end. Such exposure of the center conductor can generate significant error factors into the low current and low voltage measurements required of the probe assemblies.
Accordingly, there is a need for an improved probe assembly having a replaceable tip. More particularly, a probe assembly is needed that provides the advantages of optimal guarding and shielding of the center conductor with a probe tip that is easily replaceable. In addition, a probe assembly is desired that provides a high degree of rigidity for the mounting of a replaceable probe tip to allow for precision manipulation and placement of the tip on a DUT for taking accurate low level measurements.
It would be desirable therefore to provide an integrated approach to guard and shield systems of wafer probe stations designed to accommodate low level current and voltage measurements with sensitivities in the high attoampere and the low femtoampere regime, which is not easily feasible with presently known designs of guarding systems or shielding systems in commercial probe stations. The shield and guard system should provide electrical isolation as well as for the reduction of parasitic capacitance and noise experienced by the device under test at the conductive test surface. Excessive hysteresis associated with built up electrical charge at the test surface should also be minimized to reduce the time required for stabilizing measurement voltages to the device under test.
Measurements of low level currents in the high attoampere and low femtoampere regime are particularly susceptible to errors induced by capacitive loading, electrical discharge, and noise events which occur because of the dielectric characteristics of nonconductors in and surrounding the conductive test surface, which effects may significantly distort measurement values and limit the accuracy of low voltage and low current measurements. Poor tester and prober grounding or poorly insulated or guarded probes will contribute to electrical noise from power supplies or external circuits which may enter the probing environment and be coupled to the measurements. Additionally, offsets and drifting associated with parasitic capacitances may result in hysteresis of the current and voltage measurements producing erroneous data offsets, inaccuracies, and long measurement times. Advantageously, it would be desirable to provide an integrated approach which brings the overall wafer probe station, probe assembly, and chuck design into cooperative relationship for both guarding and shielding for the reduction of parasitics and noise and which also minimizes the effects of capacitance in the overall system.
The present invention addresses the problems associated with prior art probe stations by providing an integrated guarding and shielding approach for limiting electrical leakage currents and noise. The guarding and shielding system provides a line-of-sight electrical barrier between a shielding element and the conductive wafer support layer to both minimize leakage currents, parasitic capacitance, electromagnetic interference (EMI) and other noise sources.
Briefly summarized, the invention relates to a chuck apparatus that may be used for both room ambient and thermal probing applications for a wafer probe station in which an upper conductive layer for supporting the DUT is electrically isolated from a lower conductive chuck element by an insulator which positions the upper conductor layer above the lower chuck element and also positions an electrically isolated conductor along its periphery. A further conductive laterally extending element is provided as a shielding element wherein one or more of the lower chuck element and the peripheral conductor form an electrical barrier between the conductive test surface and the shielding element. Alternatively, the shielding element may also be connected as a guard and conductive surfaces of the probe station chassis used as a shield. Various other guarding and shielding approaches are also made possible with the novel chuck apparatus of the invention.
The shielding and guarding approaches described in accordance with one embodiment of the invention provides for the use of a lower chuck element in the form of an aluminum alloy disk or puck with cast-in heating and cooling elements and temperature sensors. In another embodiment, the lower chuck element is a metal coated ceramic disk with case-in heating and cooling elements and sensors. The temperature control elements may be omitted for room ambient applications.
The insulator supported on the lower chuck element may be provided in various geometric configurations to permit a number of arrangements of the conductive test layer, the peripheral conductive element and the lower chuck element relative to each other. The unique design is mechanically rigid, ensures proper registration of the various components over time, and provides improved performance in low voltage and low noise applications. The novel chuck assembly of the invention may be arranged in a variety of electrical connection schemes with the test instrumentation and with the probe station chassis.
The invention also relates to a probe assembly comprising a rigid triaxial probe holder designed to be connected to a replaceable probe cartridge. The probe holder is provided as an extending metal shaft having an integrated triaxial connector at one end for connecting to a test instrument using triaxial cable. First and second semi-rigid conductive members extend within the probe holder along its length and are isolated from each other and from the outer shaft by intervening dielectric material. A replaceable probe cartridge is provided having a center conductive member attached to a needle probe and an outer conductive member extending about the probe and isolated therefrom by an intervening dielectric material. The center conductive member extends beyond the cartridge at the end opposite the probe tip and is sized to mate with the conductive socket connected to the first conductive member of the probe holder to form a removable connection between the probe cartridge and probe holder. The outer conductive member of the probe cartridge also mates with the second conductive member of the probe holder to rigidly secure the probe cartridge to the holder. With this arrangement, the probe tip may be connected to the test instrument, and the outer conductor of the probe cartridge driven as a guard. The outer conductive shaft of the probe holder may be grounded to provide the fully integrated grounded and shielded probe assembly. The unique probe cartridge of the invention provides a replaceable probe tip solution and the ability to guard the probe along its length to a location very near the probe point to provide enhanced performance in low current and low voltage measurement applications. The resulting probe assembly is also mechanically rigid and facilitates fast and convenient probe replacement. The novel design may be arranged in a variety of electrical connection schemes with the test instrumentation and other components of the probe station to provide an integrated guarding and shielding approach for a wide variety of testing applications.
In another aspect of the invention, a probe assembly is provided that allows for replacement of the probe tip thereof via a detachable connection provided between the tip and a probe holder. The probe tip and probe holder include respective conductors that are positioned relative to each other when one tip is replaced with another like tip so that consistent and reliable signal transmission occurs along the pathway formed by the conductors and at the interface therebetween. Preferably, the probe holder rearward conductor includes a tubular portion having a forward end wall portion that is bent over to form a mouth for receipt of rear portion of the tip forward conductor therein. More specifically, the bent over portion extends radially inward and rearward to its annular free end to form a tapered mouth surface that guides the tip conductor into central opening bounded by the annular end when the probe tip is inserted into the probe holder. In addition, the mouth wall portion is resiliently flexible to securely and tightly grip the tip conductor rear portion when inserted therein so that there is a low resistance ohmic contact at the interface between the holder and tip signal or center conductors. In this manner, conductivity degradation at this interface between the holder and replaceable tip is kept to a minimum. This is especially important for the low level measurements that the present probe assembly takes, e.g., on the order of attoAmp signal levels of testing. Further, electrical barriers in the forms of guarding or shielding conductor members can be provided, to improve the accuracy of the measurements being taken of the DUT by way of distal tip end of the probe tip conductor. In the preferred form, the conductor extending about the probe tip extends to closely adjacent the tip end to maximize the electrical protection provided thereby. In this manner, the length of the probe tip that is not guarded is kept to a minimum for accurate signal measurements.
Preferably, the probe tip has a bent configuration for providing an angle of attack toward the DUT, and the guard member is of a metal material that extends along the bent configuration to closely adjacent the tip end so that the probe tip has rigidity and robustness along substantially its entire length. Thus, the probe tip has durability as the added strength provided by the metal guard member extending to closely adjacent the tip end thereof lessens the likelihood of tip end breakage such as due to impacts thereagainst. The rigidity of the probe tip provided by the guard member is also desirable for keeping the tip stable in proper contact with the DUT and minimizing vibrations thereof that may otherwise be generated during its operation.
In a preferred form the tip conductor rear portion includes a biased member that is urged into the mouth of the probe conductor. In this form, the forward and the rearward conductors are resiliently urged toward each other and into intimate contact at their interface along outer surface of the biased member and inner end of the mouth to maintain the tip securely connected to the probe holder to ensure against conductivity degradation that otherwise might adversely affect the accuracy of the low level measurements taken by the probe assembly.